Ben Santer on the attribution of extreme weather events to climate change

“When you warm up the planet, you experience that through changes in weather that makes up the climate,” says Dr. Benjamin Santer at the Lawrence Livermore National Lab. In a video interview with Climate Science Watch, Santer answers the questions: What is the most appropriate way for reporters and scientists to make a distinction between climate and weather when discussing the attribution of specific weather events, such as heat waves, droughts, and intense precipitation to climate change? What do you think is the most important message for the public to take away from witnessing these events?

We conducted the interview on August 27, 2010.

Santer’s written testimony before the House Committee on Science and Technology, Subcommittee on Energy and Environment, held in Washington, D.C. on November 17 includes the following within its 19 pages:

Assessing Risks of Changes in Extreme Events

We are now capable of making informed scientific statements regarding the influence of human activities on the likelihood of extreme events (75, 76, 77).

As noted previously, computer models can be used to perform the control experiment (no human effects on climate) that we cannot perform in the real world. Using the “unforced” climate variability from a multi-century control run, it is possible to determine how many times an extreme event of a given magnitude should have been observed in the absence of human interference. The probability of obtaining the same extreme event is then calculated in a perturbed climate – for example, in a model experiment with historical or future increases in greenhouse gases, or under some specified change in mean climate (78). Comparison of the frequencies of extremes in the control and perturbed experiments allows climate scientists to make probabilistic statements about how human-induced climate change may have altered the likelihood of the extreme event (53, 78, 79). This is sometimes referred to as an assessment of “fractional attributable risk” (78).

Recently, a “fractional attributable risk” study of the 2003 European summer heat wave concluded that “there is a greater than 90% chance that over half the risk of European summer temperatures exceeding a threshold of 1.6 K is attributable to human influence on climate” (78).

This study (and related work) illustrates that the “D&A” [detection and attribution] community has moved beyond analysis of changes in the mean state of the climate. We now apply rigorous statistical methods to the problem of estimating how human activities may alter the probability of occurrence extreme events. The demonstration of human culpability in changing these risks is likely to have significant implications for the debate on policy responses to climate change.

Also see the discussion of climate and weather extremes in the written testimony (pp. 10-13) submitted for the hearing record by Dr. Heidi Cullen and the written testimony (pp. 6-9) of Dr. Gerald Meehl.